In general, the production of para-propylbenzaldehyde has been hampered by low yields and/or requirements that hazardous reactants and reagents be used at high pressures. Processes such as the Duff Formylation (formylation of propylbenzene with hexamine in trifluoroacetic acid to give para-propylbenzaldehyde) result in relatively low yields of the para-isomer. The formylation of an alkylbenzenes with carbon monoxide in the presence of hydrogen chloride and aluminum chloride to give para-propylbenzaldehyde was first reported by Gattermann and Koch in 1897. (Gattermann and Koch, Chemische Berichte, 1897, 30, 1622.) The reaction, known as the “Gattermann-Koch” reaction, has been reviewed by Crounse in Organic Reactions, 1949, 5, 290 and by Olah and Huhn in Friedel-Crafts and Related Reactions, 1964, Vol. 3, Part 2, 1153. Use of the Gattermann-Koch reaction with hydrogen chloride and aluminum trichloride generally gives an increased production of para-product with respect to ortho and meta isomers. Other processes, such as the Gattermann-Koch carbonylation of propylbenzene with carbon monoxide, in this case, in the presence of hydrogen fluoride and boron trifluoride, require the use of hazardous compounds, i.e., hydrogen fluoride and boron trifluoride at high pressures.
Alkylbenzenes have been observed to undergo disproportionation (alkylation and dealkylation) in the presence of aluminum chloride. In the disproportionation reaction, the catalyst causes the transfer of the alkyl substituent of the propylbenzene substrate to another aromatic molecule, such as a molecule of propylbenzene reactant, giving aromatic impurities such as dipropylbenzene and benzene. Tripropylbenzene impurities can also be formed. Formylation product may also undergo disproportionation, giving additional impurity species. These byproduct impurities are generally formed at the expense of the desired formylated para product, thus giving a reduced yield of the desired product. The disproportionation of alkylbenzene in the presence of aluminum chloride is generally known in the art and has been reported by Crounse, J. Am. Chem. Soc., 1949, 71, 1263 and by Baddeley and Kenner, J. Chem. Soc., 1935, 303.
The Gattermann-Koch reaction has been conducted in various aromatic solvents such as benzene, nitrobenzene, chlorobenzene, and halogenated hydrocarbons, or an excess of the aromatic compound to be formylated. U.S. Pat. No. 4,622,429 discloses relatively low yields of formylation product when benzene is used as solvent. U.S. Pat. No. 4,195,040 and German Patent DE403,489 employ chlorobenzene and nitrobenzene, respectively as solvents. Instead of excess aromatic reactant, benzene has been used to some extent in reducing the effects of disproportionation and increasing the yields of alkylbenzaldehyde. In spite of these efforts, a need exists for a Gatterman-Koch-type formylation reaction which reduces the incidence of disproportionation product, but which allows for improved para-product selectivity with respect to current methods.